Return srclen in proc.idMapFileOperations.Write.

PiperOrigin-RevId: 241037926
Change-Id: I4b0381ac1c7575e8b861291b068d3da22bc03850

1 files changed, 4 insertions, 1 deletions

diff --git a/pkg/sentry/fs/proc/uid_gid_map.go b/pkg/sentry/fs/proc/uid_gid_map.go
index a52e0cb1f..0c68bbfc9 100644
--- a/pkg/sentry/fs/proc/uid_gid_map.go
+++ b/pkg/sentry/fs/proc/uid_gid_map.go
@@ -169,5 +169,8 @@ func (imfo *idMapFileOperations) Write(ctx context.Context, file *fs.File, src u
 	if err != nil {
 		return 0, err
 	}
-	return int64(len(b)), nil
+
+	// On success, Linux's kernel/user_namespace.c:map_write() always returns
+	// count, even if fewer bytes were used.
+	return int64(srclen), nil
 }